In recent years, photochemical reactions, in which a solution of an organic compound is irradiated with ultraviolet radiation to cause a chemical reaction of the organic compound solution, thereby forming another compound from the organic compound, have been conducted in a field of chemical syntheses. The syntheses of, for example, 6-nylon, benzene hexachloride and other compounds have been conducted by utilizing photochemical reactions (Reference “Yuki Gosei Kagaku (Synthetic Organic Chemistry)”, written by Hirotada Iida, published by Baifukan, pages 278 and 198).
FIG. 6 is a perspective view illustrating an exemplary ultraviolet irradiation apparatus for such photochemical reactions. In this apparatus, a reaction vessel 4 is made of Pyrex glass, an inner tube 3 is inserted and arranged in this reaction vessel 4 so as to construct a double tube structure together with the reaction vessel 4, a long arc type high pressure mercury lamp 1 is inserted as a light source in the inner tube 3, and a cylindrical Vycor filter 5 is arranged within the inner tube 3 so as to surround the periphery of the high pressure mercury lamp 1.
In this apparatus, a photo-reactive solution is charged into a cylindrical space between the reaction vessel 4 and the inner tube 3 and irradiated with ultraviolet radiation emitted from the high pressure mercury lamp 1 through the Vycor filter 5 in a state that cooling water for cooling the high pressure mercury lamp 1 is running through the inner tube 3 via a cooling water inlet 8 and a cooling water outlet 9 formed at the upper part of the inner tube 3 or in a state that the photo-reactive solution charged into the cylindrical space between the reaction vessel 4 and the inner tube 3 is stirred by, for example, a magnetic stirrer 7 or a stirring rod, or is bubbled with an inert gas such as argon gas introduced through a gas inlet tube 6.
However, the ultraviolet irradiation apparatus of such a construction has involved problems that, since the ultraviolet radiation emitted from the high pressure mercury lamp 1 is struck on the photo-reactive solution through the inner tube 3 necessary for forming a flow path for the cooling water, the intensity of the ultraviolet radiation actually applied to the photo-reactive solution attenuates to become low, and that the ultraviolet radiation emitted from the high pressure mercury lamp 1 is a line spectrum over a wide wavelength range, and so the photo-reactive solution cannot be always irradiated with ultraviolet rays having a wavelength optimum for the intended photochemical reaction at a high efficiency even when the Vycor filter 5 having wavelength selectivity is used.
Here, typical examples of the photochemical reaction include photochemical reactions for synthesizing, for example, vitamin D derivatives. The vitamin D derivatives have been known to be useful as medicines for osteoporosis, hyperparathyroidism, psoriasis, etc.
As a process for synthesizing a vitamin D derivative, there has heretofore been known a process comprising irradiating its corresponding provitamin D derivative with ultraviolet radiation emitted from a high pressure mercury lamp of such an ultraviolet irradiation apparatus as described, for example, above through a Vycor filter or the like and further subjecting the previtamin D derivative obtained by this reaction to a thermal isomerization reaction. However, according to this process, the yield of the intended vitamin D derivative is as low as several percent to ten-odd percent because the yield of the photochemical reaction is low. This fact is as described in, for example, Japanese Patent Application Laid-Open No. 188061/1991, 72994/1994 or 80626/1994.
On the other hand, in order to synthesize a previtamin D derivative, it has also been known to utilize a two-step process of reaction in which a photo-reactive solution is irradiated with a monochromatic laser beam in place of the light from a high pressure mercury lamp having no specificity to obtain a tachysterol derivative as an intermediate product, and the tachysterol derivative is further irradiated with a laser beam having a different wavelength (J. Am. Chem. Soc., Vol. 103, p. 6781 (1981) and Japanese Patent Application Laid-Open Nos. 89473 to 89476/1992).
However, this process is poor in productivity because the efficiency is low due to the use of the laser beams, and hence cannot be practically used on an industrial scale.
In addition, as a two-step process of reaction, it has been reported to cut rays of a wavelength range of 300 to 315 nm among rays emitted from a high pressure mercury lamp by an organic compound typified by a dimethylaminobenzoate, thereby inhibiting the formation of a lumisterol derivative. However, it is necessary to use a photosensitizer for the purpose of converting a tachysterol derivative obtained as an intermediate product into a previtamin D derivative (J. Org. Chem., Vol. 60, p. 767 (1995)). Accordingly, a process for removing it becomes a great problem as a production process of a medical drug.
Besides, it has also been known to use a solution filter upon irradiation of light so as to conduct the irradiation of the light with the wavelength of the light limited. The yield thereof is about 40%, and so a great improvement cannot be expected, and this process also involves a problem from the viewpoint of waste disposal of the compound used as the solution filter (J. Nutr. Sci. Vitamino., Vol. 26, p. 545 (1980)).
On the other hand, in the above-described reaction of the one-step process of light irradiation, the most effective light is known to be ultraviolet rays having a wavelength of 295 nm (J. Am. Chem. Soc., Vol. 104, p. 5780 (1982) and J. Am. Chem. Soc., Vol. 110, p. 2548 (1988)).
Examples of an electric discharge lamp emitting ultraviolet rays within a wavelength range including wavelengths around 295 nm in high intensity, include super high pressure mercury lamps and xenon-mercury lamps. However, since these lamps are short arc type lamps of point light source, their emission length is short, and so such a lamp fails to irradiate the photo-reactive solution sufficiently over the whole region within the reaction vessel 4 with ultraviolet radiation when the lamp is arranged in the interior of the inner tube making up the double tube structure like the apparatus of the construction illustrated in FIG. 6.
When a wavelength-selective filter is used, ultraviolet rays having a required wavelength can be provided. However, an interference filter for selectively taking the ultraviolet rays having the required wavelength out of continuous spectrum light is limited in size to about 100 mm in terms of its diameter for reasons of production. Therefore, such a large-sized filter for covering the whole of an electric discharge lamp such as a super high pressure mercury lamp or xenon-mercury lamp can not be provided.